1. Field of the Invention
The present invention generally relates to materials for photovoltaic cells. More particularly, it relates to a method for preparing copper indium gallium diselenide/disulfide (GIGS) nanoparticles. The invention further relates to CIGS-based devices formed from nanoparticle-based precursor inks.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
For commercial viability, photovoltaic (PV) cells must generate electricity at a competitive cost to fossil fuels. To meet these costs, the PV cells must comprise low cost materials along with an inexpensive device fabrication process and with moderate to high conversion efficiency of sunlight to electricity. For a device-building method to succeed, the materials synthesis and device fabrication must be commercially scalable.
The current photovoltaic market remains dominated by silicon wafer-based solar cells (first-generation solar cells). However, the active layer in these solar cells is made of silicon wafers having a thickness ranging from microns to hundreds of microns because silicon is a relatively poor absorber of light. These single-crystal wafers are very expensive to produce because the process involves fabricating and slicing high-purity, single-crystal silicon ingots, and is also very wasteful.
The high cost of crystalline silicon wafers has led the industry to look at cheaper materials to make solar cells and for this reason much development work has focused on producing high efficiency thin film solar cells where material costs are significantly reduced compared to silicon.
Semiconductor materials like copper indium gallium diselenides and disulfides (Cu(In,Ga)(S,Se)2, herein referred to as “GIGS”) are strong light absorbers and have band gaps that match well with the optimal spectral range for PV applications. Furthermore, because these materials have strong absorption coefficients, the active layer in the solar cell is required to be only a few microns thick.
Copper indium diselenide (CuInSe2) is one of the most promising candidates for thin-film PV applications due to its unique structural and electrical properties. Its band gap of 1.0 eV is well matched with the solar spectrum. CuInSe2 solar cells can be made by the selenization of CuInS2 films because, during the selenization process, Se replaces S and the substitution creates volume expansion, which reduces void space and reproducibly leads to high-quality, dense, CuInSe2 absorber layers. [Q. Guo, G. M. Ford, H. W. Hillhouse and R. Agrawal, Nano Lett., 2009, 9, 3060] Assuming complete replacement of S with Se, the resulting lattice volume expansion is ˜14.6%, which is calculated based on the lattice parameters of chalcopyrite (tetragonal) CuInS2 (a=5.52 Å, c=11.12 Å) and CuInSe2 (a=5.78 Å, c=11.62 Å). This means that the CuInS2 nanocrystal film can be easily converted to a predominantly selenide material, by annealing the film in a selenium-rich atmosphere. Therefore, CuInS2 is a promising alternative precursor for producing CuInSe2 or CuIn(S,Se)2 absorber layers.
The theoretical optimum band gap for absorber materials is in the region of 1.2-1.4 eV. By incorporating gallium into CuIn(S,Se)2 nanoparticles, the band gap can be manipulated such that, following selenization, a CuxInyGazSaSeb absorber layer is formed with an optimal band gap for solar absorption.
Conventionally, costly vapor phase or evaporation techniques (for example metal-organic chemical vapor deposition (MO-CVD), radio frequency (RF) sputtering, and flash evaporation) have been used to deposit CIGS films on a substrate. While these techniques deliver high-quality films, they are difficult and expensive to scale to larger-area deposition and higher process throughput.
One of the major advantages of using nanoparticles of CIGS is that they can be dispersed in a medium to form an ink that can be printed on a substrate in a similar way to inks in a newspaper-like process. The nanoparticle ink or paste can be deposited using low-cost printing techniques such as spin coating, slit coating and doctor blading. Printable solar cells could replace the standard conventional vacuum-deposited methods of solar cell manufacture because the printing processes, especially when implemented in a roll-to-roll processing framework, enables a much higher throughput.
The synthetic methods developed so far offer limited control over the particle morphology, and particle solubility is usually poor which makes ink formulation difficult.
The challenge is to produce nanoparticles that overall are small, have a low melting point, narrow size distribution and incorporate a volatile capping agent, so that they can be dispersed in a medium and the capping agent can be eliminated easily during the film baking process. Another challenge is to avoid the inclusion of impurities, either from synthetic precursors or organic ligands that could compromise the overall efficiency of the final device. The applicant's co-pending U.S. patent applications published as Nos. 2009/0139574 and 2014/0249324 describe the scalable synthesis of CIGS nanoparticles capped with organo-chalcogen ligands for use as precursors for the formation of photovoltaic devices, and are hereby incorporated by reference in their entireties.
One of the challenges associated with the nanoparticle-based CIGS deposition approach is to achieve large grains after thermal processing. Grain sizes on the order of the film thickness are desirable since grain boundaries act as electron-hole recombination centers.
Elemental dopants, such as sodium [R. Kimura, T. Mouri, N. Takuhai, T. Nakada, S. Niki, A. Yamada, P. Fons, T. Matsuzawa, K. Takahashi and A. Kunioka, Jpn. J. Appl. Phys., 1999, 38, L899] and antimony, [M. Yuan, D. B. Mitzi, W. Liu, A. J. Kellock, S. J. Chey and V. R. Deline, Chem. Mater., 2010, 22, 285] have been reported to enhance the grain size of CIGS films and thus the power conversion efficiency (PCE) of the resulting devices.
In another approach, a binary copper chalcogenide compound is added the CIGS precursor(s) to promote grain growth. Copper chalcogenide compounds with a lower melting point than CIGS can act as a sintering flux to promote grain growth of the CIGS layer at a temperature well-below its melting point. It is thought that, at the sintering temperature, the sintering flux is a liquid that wets the CIGS grains such that they dissolve in the liquid. This is believed to promote particle bonding, leading to higher densification rates and lower sintering temperatures, and is referred to as “liquid phase sintering.”
Casteleyn et al. studied the influence of Cu, CuSe, Cu2Se and Se additives on CuInSe2 films. [M. Casteleyn, M. Burgelman, B. Depuydt, A. Niemegeers and I. Clemminck, IEEE First World Conference on Photovoltaic Energy Conversion, 1994, 1, 230]. It was found that, in a selenium-rich atmosphere, Cu-rich phases formed CuSe, which acted as a flux above its melting point (523° C.) to promote liquid phase sintering and thus promote grain growth.
Kim et al. applied a layer of sputtered Cu2Se onto the surface of a sputtered Cu—In—Ga film that was subsequently selenised to form a CIGS layer. [M. S. Kim, R. B. V. Chalapathy, K. H. Yoon and B. T. Ahn, J. Electrochem Soc., 2010, 157, B154] Cu2Se was found to promote grain growth when the overall Cu/(In+Ga) ratio was greater than 0.92.
Cu2S powder (melting point: 435° C.) has been added to Cu2In2O5 nanoparticles prior to sulfurization, to promote the conversion to chalcopyrite CuInS2 and facilitate grain growth. [C.-Y. Su, D. K. Mishra, C.-Y. Chiu and J.-M. Ting, Surf. Coat. Technol., 2013, 231, 517]
The applicant's U.S. patent application No. 61/847,639 describes the preparation of copper selenide nanoparticles, which can be added to CIGS materials as a flux to promote grain growth. The nanoparticle melting point is lower than that of the corresponding bulk copper selenide phase, enabling copper selenide nanoparticles to effect liquid phase sintering at a reduced temperature.
In the prior art methods to promote grain growth using binary copper chalcogenide precursors, a pre-fabricated copper chalcogenide compound is employed. Thus, there is a need for a method to form the copper chalcogenide phase in situ, to reduce the processing requirements associated with the formation of a CIGS film with large grains. Herein, a method is described to enhance the grain size of CIGS films using a nanoparticle-based deposition approach, wherein the CIGS nanoparticles are copper-rich. The nanoparticles can be processed in a chalcogen-rich atmosphere to facilitate conversion of the excess copper to copper selenide or copper sulfide that acts as a sintering flux to promote liquid phase sintering and thus the growth of large grains. The stoichiometry of the resulting CIGS absorber layer can be controlled by both the nanoparticle stoichiometry and post-annealing processing steps, such as KCN etching.